Controlled Generation and Understanding of Defects in 2D Materials

The understanding of defects in two-dimensional (2D) materials, such as semiconducting transition metal dichalcogenides (TMDs), is key to exploit their properties for applications in electronics, optoelectronics, and catalysis, among others.

Defects in the form of dopants were introduced in TMD monolayers through the development of a liquid-phase precursor assisted technique. Substitutional dopants in MoS2 and WS2 layers were identified by high-resolution scanning transmission electron microscopy (HR-STEM). This synthesis approach can be tuned to control the doping concentration in the samples up to very high levels (more than 10 at%). The consequent changes in the photoluminescence (PL) emission and Raman spectra show that the proposed synthesis route is effective for engineering functionalities in TMD layers.

Moreover, we induced defects in MoS2 monolayers through controlled gallium ion irradiation, and carried out defect-healing experiments. HR-STEM and Raman spectroscopy characterizations allowed for an estimation of the distance between defects as a function of the longitudinal acoustic (LA) and A’1 modes’ intensities. Temperature dependent Raman spectroscopy studies of MoS2 samples with native defects were also conducted with various excitation energies. The analysis of defect induced Raman modes shows a clear dependence on temperature and excitation energy, as well as a linear trend between the density of defects and the relative intensity of the LA and transversal acoustic (TA) modes with respect to the first-order E' modes. An expression for the quantification of defects in MoS2 was also established at various laser energies across excitonic transitions.

In summary, native and induced defects in TMDs were studied through a combination of electron microscopy, Raman, and PL spectroscopies. These results represent a contribution towards the manipulation, quantification and understanding of defects in TMDs, bringing 2D materials one step closer to applications.